Field of the Invention
The present invention relates to a pneumatic tire in which a sipe is formed in a land portion of a tread surface, and is more particularly useful as a studless tire.
Description of the Related Art
Conventionally, a cut called as a sipe has been formed in a land portion such as a block and a rib, in the studless tire. Due to an edge effect and a water removing effect of the sipe, traveling on an ice road surface having a low coefficient of friction can be stabilized, whereby it is possible to enhance a so-called ice performance. A two-dimensional sipe formed into a two-dimensional shape which is not changed along a depth direction has been known as the sipe mentioned above, and a plane sipe and a wavy sipe are put to practical use.
Further, a three-dimensional sipe formed into a three-dimensional shape which is changed along a depth direction has been known, as described in Patent Documents 1 to 6. In the three-dimensional sipe, excessive deformation of the land portion is suppressed since wall surfaces of the sipe are engaged with each other at the braking time and the cornering time. As a result, it is possible to securely achieve the edge effect and the water removing effect. However, if the three-dimensional shape is uniformly formed, there is a case that only the deformation in a certain specific direction (for example, a back and forth direction) is suppressed. There has been a risk that wear resistance and irregular wear resistance are lowered since the deformation in the other direction (for example, a lateral direction) can not be sufficiently suppressed.
On the other hand, it has been found that there is room for improvement in the ice performance only by employing the three-dimensional shape which can suppress the deformation in a plurality of directions (for example, in the back and forth direction and the lateral direction). According to a study by the inventor of the present invention, deformability of the land portion is differentiated by positions of the sipe, and is differentiated by wear stages of the land portion. A specific sipe structure taking them into consideration has not been known.